Power inverters convert a DC power to an AC power. For example, some power inverters are configured to convert the DC power to an AC power suitable for supplying energy to an AC grid and, in some cases, an AC load coupled to the AC grid. One particular application for such power inverters is the conversion of DC power generated by an alternative energy source, such as photovoltaic cells (“PV cells” or “solar cells”), fuel cells, DC wind turbine, DC water turbine, and other DC power sources, to a single-phase AC power for delivery to the AC grid at the grid frequency.
A basic electrical property of a single-phase AC power system is that the energy flow includes both an average power portion that delivers useful energy from the energy source to the load and a double-frequency portion that flows back and forth between the load and the source:p(t)=Po+Po*cos(2ωt+φ)  Equation 1
In applications involving inverters, the double-frequency portion represents undesirable ripple power that, if reflected back into the DC power source in a significant amount, may compromise performance of the source. Such concerns are particularly relevant to photovoltaic cells wherein the power delivered by each photovoltaic cell may vary in magnitude over time due to temporal variations in operating conditions including changes in sunlight intensity, angle of incidence of sunlight, ambient temperature and other factors. As such, photovoltaic cells have an operating point at which the values of the current and voltage of the cell result in an ideal or “maximum” power output. This “maximum power point” (“MPP”) is a function of environmental variables, including light intensity and temperature. Inverters for photovoltaic systems may include some form of maximum power point tracking (“MPPT”) as a mechanism of identifying and tracking the maximum power point (“MPP”) and adjusting the inverter to exploit the full power capacity of the cell at the MPP. Extracting maximum power from a photovoltaic cell requires that the cell operate continuously at its MPP. As such, fluctuations in power demand, caused, for example, by the double-frequency ripple power being reflected back into the cell, may compromise the ability of the inverter to deliver the cell's maximum power.
In typical inverters, the double-frequency ripple power is managed by storing and delivering energy at twice the AC frequency. To do so, a passive or active filter is typically used to manage the double-frequency ripple power on the input side of the inverter. In passive filtering arrangements, a large capacitance is typically required because the capacitive device must support the DC bus voltage without imposing significant voltage ripple on the DC bus. However, in active filter arrangements, a relatively smaller capacitance may be used because the capacitive device need not support the DC bus voltage. Because the active filter “isolates” the internal capacitor from the DC bus, the voltage variation across the internal capacitor can be relatively large and the value of the capacitor may be made relatively small.
In a typical photovoltaic power system, an inverter may be associated with one or more solar cell panel. For example, some systems include strings of solar cell panels that deliver a relatively high, combined voltage (e.g., nominal 450 V) to a single, large inverter. Alternatively, in other systems such as a distributed photovoltaic power system, an inverter may be associated with each solar cell panel. In such systems, the solar cell panels are typically small, relatively low voltage (e.g., 25 V). The inverter may be placed in close proximity to the associated solar cell panel to increase the conversion efficiency of the overall system.